Using some waterproof paint and sponges, stamp print designs on a clear or colored shower curtain liner. You can cut sponges into shapes like stars, trees, etc and using different colors of paint, stamp the designs onto the liner.

I thought of a cute idea...buy a shower curtain liner for $3.00. and get your favorite christmas fabric, and sew it to the liner. But the regular shower curtain measures 74x74, so you would have to put a different fabric around the perimeter for border. That would be really cute.